The field of the invention is in the ink jet printing art, and more particularly in that of charge plates having individual jet charging electrodes for ink jet printing heads such as the general type described in Beam et al. U.S. Pat. No. 3,586,907. Coating heads of that type are used in ink jet printing systems, which create printed matter by selective charging, deflecting, and catching of drops produced by the stimulation of one or more rows of continuous flowing ink jets. The jets themselves are produced by forcing ink under pressure through a series of orifices in an orifice plate, which is one component of the coating head.
A stimulation arrangement stimulates the jets to break the jets up into uniformly sized and regularly spaced drops, with drop formation occuring in all jets at more or less fixed positions, all located approximately the same distance from the orifice plate. The charge plate is positioned within the coating head so as to achieve electrical charging of selected ones of the drops being generated.
A charge plate as taught in the Beam et al. patent comprises a plate of dielectric material provided with a series of charging tunnels located equidistantly along a straight line. Each charging tunnel is coated with an electrically conductive material so as to define a cylindrical charging electrode. Electrical leads are connected to each such charge electrode, and the electrical leads in turn are activated selectively by an appropriate data processing system. Typical prior art charge plates including such electrodes are disclosed in Solyst U.S. Pat. No. 3,975,741; in Kuhn U.S. Pat. No. 3,984,843; and in Bassous et al. U.S. Pat. No. 4,047,184. The prior art also includes charge plates having charging electrodes formed in notches along the edges of the plate as disclosed in the above-mentioned Solyst patent, and also in Culp U.S. Pat. No. 3,618,858.
The charging electrodes whether they be tunnels, slots, or flat faces all present very difficult fabrication problems because of the many requirements imposed upon them. The charging electrodes are quite small (typical tunnel-type electrodes have a 0.012 inch hole diameter). To provide the desired interchangeability of charging plates, the charge electrodes must be accurately positioned not only with respect to each other but also with respect to the confines of the entire charge plate. The desired printing or coating characteristics demanded of this system require that the charging electrodes is located extremely close together. Another requirement of the charging electrodes, as used in the ink jet system, is that they have a relatively high length to cross section ratio. In addition to the foregoing requirements the charge plates must have a high degree of dimensional stability, be rugged and resistant to the ink or coating fluid used in the printing or coating system in which they are placed. Electrically, the charging electrodes must be well insulated from each other with typical values of insulation resistance being greater than 10.sup.10 ohms. Provision must be available for conveniently making electrical contact with the charging electrodes. The resistance from the contact region throughout the electrode surface should not be greater than a couple of ohms.
Many attempts in the prior art have been made to provide a charging plate that will meet the foregoing desired requirements. Drilling of the tunnels in printed circuit boards and machinable ceramics is extremely expensive in materials having sufficient rigidity. Casting of charge plates has not been satisfactory due to the poor dimensional stability of known suitable casting materials for this application. Laser drilling of various suitable substrates has also been attempted. Generally, all the foregoing prior art charging plates have had the problems of the charging electrodes having holes or surfces that were rough, dimensional accuracy varied more than desirable, and the holes were either excessively bell-shaped or tapered. Metalization of the circuit and making satisfactory electrical connections has been difficult. To provide the desired physical resistance to the corrosiveness of the ink coating fluid, a covering material was frequently required. In addition, the prior art charge plates have been relatively fragile, too fragile for ease of handling during changing and cleaning.
Another attempt in the prior art to provide a suitable charge plate is shown in the disclosure by James L. Vedder in the publication "Research Disclosure" for January 1978 at Page 10; published by Industrial Opportunities Ltd., Homewell, Havant, Hampshire, P091EF, U.K. His charge plate is etched from solid metal sheet. Providing suitable charge tunnels in materials having the required thickness for proper operation of the systems of the type herein considered generally has not been completely satisfactory.
Generally, the most satisfactory charge plate of the non-laminated type thus far used for such purposes has been photofabricated from a photosensitive ceramic material which is exposed, etched, and thereafter fired to a final state. It has been found that the firing process causes dimensional alteration of the plate so that the yield of the plates is poor. Those plates which do pass inspection are in many cases marginally acceptable, and they are in any event quite delicate and easily damaged. These plates require the formation of charge tunnels using photolithography and thin film techniques. The circuitry formed is labor intensive and the corrosion resistance of these leads is poor.
Laminated charge plates are well known. Typical prior art laminated charge plates are illustrated by the previously mentioned Solyst in U.S. Pat. No. 3,975,741 and further by Olsen et al. in U.S. Pat. No. 4,096,626 and Paranjpe et al. in U.S. Pat. No. 4,223,320. All of the known prior art laminated charge plates are composed of laminations of dielectric materials. Generally, these charge plates have the previously mentioned problems of dimensional accuracy, dimensional stability, and difficulty of making a satisfactory electrical circuit.
Japanese patent application No. 1222214/1975, patent laid open No. 46724/1977 entitled "Method of Preparing Printing Conductor in Electronic Printer" is of interest in that it teaches the formation of printed circuitry by etching a thin sheet of metal to provide conductors having lead lines and connecting regions. The lamination of conductors is not involved nor is a charge plate for ink jet printers suggested.